An ultrasonic sensing device is widely used for measuring a relative distance or detecting whether an object is within the sensing range of the ultrasonic sensing device. According to the measuring or detecting result, further actions will be performed.
Generally, when the ultrasonic sensing device is activated, a sensing wave is generated to detect whether any object is within the sensing range of the ultrasonic sensing device. In a case that an object enters the sensing range of the ultrasonic sensing device, the sensing wave is reflected by the object and the reflected sensing wave (also referred as an echo signal) is returned back to the ultrasonic sensing device. When the echo signal is received, the ultrasonic sensing device will calculate the time interval between generation of the sensing wave and receipt of the echo signal, thereby acquiring a time of flight (TOF). According to the time of flight (TOF), the ultrasonic sensing device could estimate the distance between the ultrasonic sensing device and the object. Alternatively, according to the time of flight (TOF), further actions will be performed.
FIG. 1A is a schematic diagram illustrating a conventional ultrasonic sensing device. As shown in FIG. 1A, the ultrasonic sensing device 10 is mounted on a supporting object 21 (e.g. a ceiling or a vehicle). A reference object 22 (e.g. a floor, a desk surface or a wall) is within the sensing range of the ultrasonic sensing device 10. The ultrasonic sensing device 10 could be used to detect whether a foreign object enters the range between the ultrasonic sensing device 10 and the reference object 22, thereby performing further actions. The operating principles of the ultrasonic sensing device 10 will be illustrated in more details as follows with reference to FIG. 1.
First of all, the reference object 22 is detected by the ultrasonic sensing device 10. Generally, after the ultrasonic sensing device 10 is activated, the sensing wave is generated, the echo signal from the reference object 22 is detected, and the time interval between generation of the sensing wave and receipt of the echo signal is calculated. As such, a reference time of flight is acquired. Next, the ultrasonic sensing device 10 periodically generates the sensing wave and receives the echo signal. If the time of flight of the echo signal received by the ultrasonic sensing device 10 is equal to the reference time of flight, the ultrasonic sensing device 10 will discriminate that the echo signal is reflected by the reference object 22. Under this circumstance, no action is done. On the other hand, in a case that a foreign object enters the range between the ultrasonic sensing device 10 and the reference object 22, the sensing wave 11 generated by the ultrasonic sensing device 10 will be reflected back by the foreign object. As such, the time of flight of the echo signal received by the ultrasonic sensing device 10 is not equal to the reference time of flight. Meanwhile, the ultrasonic sensing device 10 discriminates that a foreign object enters the sensing range of the ultrasonic sensing device 10, and further actions are performed.
As known, in a case that the undesired noise is received, erroneous discrimination of the ultrasonic sensing device occurs. In order to prevent from erroneous discrimination, a boundary value is usually predetermined in the ultrasonic sensing device. Once the intensity of the echo signal is greater than the predetermined boundary value, the ultrasonic sensing device will record the time of receiving the echo signal. According to the time of receiving the echo signal, the time of flight will be calculated.
Moreover, after the reference object is detected and the reference time of flight is recorded, the ultrasonic sensing device is in a detecting status. Once the ultrasonic sensing device is operated in the detecting status, the ultrasonic sensing device periodically generates a sensing wave in a predetermined detecting cycle, receives a corresponding effective echo signal, and calculates the time of flight. In such way, the ultrasonic sensing device could determine whether any foreign object enters the sensing range of the ultrasonic sensing device. However, if a multiple reflection effect of the sensing wave occurs, the ultrasonic sensing device fails to actually discriminate whether any foreign object enters the sensing range of the ultrasonic sensing device.
FIG. 1B is a schematic timing waveform diagram of the sensing wave once the multiple reflection effect occurs. After the sensing wave 11 is generated by the ultrasonic sensing device 10, a main echo signal 12 is reflected by the reference object 22 and then received by the ultrasonic sensing device 10 at the time t0. During the main echo signal 12 is returned back to the ultrasonic sensing device 10, the main echo signal 12 also hits the supporting object 21. The main echo signal 12 is reflected by the supporting object 21, moved downwardly to hit the reference object 22, and reflected back to the ultrasonic sensing device 10 again. Consequently, at the time t1, a first reflected echo signal 13 is received by the ultrasonic sensing device 10. During the first reflected echo signal 13 is returned back to the ultrasonic sensing device 10, the first reflected echo signal 13 also hits the supporting object 21. The first reflected echo signal 13 is reflected by the supporting object 21, moved downwardly to hit the reference object 22, and reflected back to the ultrasonic sensing device 10 again. Consequently, at the time t2, a second reflected echo signal 14 is received by the ultrasonic sensing device 10. Similarly, a third reflected echo signal 15 is received by the ultrasonic sensing device 10 at the time t3, and a fourth reflected echo signal 16 is received by the ultrasonic sensing device 10 at the time t4.
As known, the intensity of the reflected echo signal is gradually decreased. As shown in FIG. 1B, the intensities of the third reflected echo signal 15 and the fourth reflected echo signal 16 are lower than the boundary value, so that the third reflected echo signal 15 and the fourth reflected echo signal 16 are ignored by the ultrasonic sensing device 10. Since the intensities of the first reflected echo signal 13 and the second reflected echo signal 14 are still greater than the boundary value, the first reflected echo signal 13 and the second reflected echo signal 14 are deemed as effective echo signals. Since the reflected echo signals having the intensity greater than the predetermined boundary value of the ultrasonic sensing device 10 are deemed as effective echo signals, the ultrasonic sensing device 10 may erroneously discriminate that a foreign object enters the sensing range.
FIG. 2 is a schematic timing waveform diagram illustrating occurrence of an erroneous discrimination of the ultrasonic sensing device. During the process of detecting a foreign object by the ultrasonic sensing device, the ultrasonic sensing device continuously generates the sensing wave in a detecting cycle T. After a first sensing wave 111 has been generated by the ultrasonic sensing device for the detecting cycle T, a second sensing wave 121 is generated by the ultrasonic sensing device. When the first sensing wave 111 hits the reference object 22, a first main echo signal 112 is reflected by reference object 22 and then received by the ultrasonic sensing device. Due to occurrence of the multiple reflection effect, the first main echo signal 112 results in a first reflected echo signal 113, a second reflected echo signal 114, a third reflected echo signal 115 and a fourth reflected echo signal 116. Since the intensities of the third reflected echo signal 115 and the fourth reflected echo signal 116 are lower than the predetermined boundary value of the ultrasonic sensing device, the third reflected echo signal 115 and the fourth reflected echo signal 116 are ignored. Since the intensities of the first reflected echo signal 113 and the second reflected echo signal 114 are still greater than the boundary value, the first reflected echo signal 113 and the second reflected echo signal 114 are deemed as effective echo signals. Similarly, when the second sensing wave 121 hits the reference object 22, a second main echo signal 122 is reflected by reference object 22 and then received by the ultrasonic sensing device. Due to occurrence of the multiple reflection effect, the second main echo signal 122 results in the reflected echo signals 123 and 124.
Generally, the time of flight is calculated according to the effective echo signal first received after the sensing wave is generated. As shown in FIG. 2, the first main echo signal 112 is the effective echo signal first received after the first sensing wave 111 is generated, and the second reflected echo signal 114 is the effective echo signal first received after the second sensing wave 121 is generated. In reality, the actual effective echo signal of the second sensing wave 121 is the second main echo signal 122, rather than the second reflected echo signal 114. In other words, since the time interval between generation of the second sensing wave 121 and receipt of the second reflected echo signal 114 is shorter than the actual time interval, the time of flight is erroneously calculated. Under this circumstance, the ultrasonic sensing device may erroneously discriminate that a foreign object enters the sensing range.